1. Field of the Invention
The present invention relates to a MEMS structure and a method for fabricating the same, and more particularly to a MEMS structure in which a floating space for allowing a movable portion to move is formed by a dry etching process, and thus is capable of fundamentally preventing a stiction phenomenon which occurs when drying an etchant or other liquid, securing electrical properties and stability thereof, and preemptively preventing electrical short circuit due to stiction; and a method for fabricating the same.
2. Description of the Related Art
Recently, a number of products employing Micro-Electro Mechanical Systems (MEMS) technology which is advantageous for miniaturization and high accuracy has been developed. To this MEMS technology are applied various techniques for fabricating a MEMS structure having a stationary portion fixed in a position by a variety of methods, and a movable portion moving relative to the stationary portion.
As examples of apparatuses having such a MEMS structure, there may be mentioned accelerometers, pressure sensors, flow sensors, transducers, microactuators and the like, manufactured by MEMS manufacturing techniques including photolithography, thin film vapor deposition, bulk micromachining, surface micromachining and etching.
Meanwhile, in order to allow the movable portion constituting a part of the MEMS structure to be moved, it is necessary to release a movable portion from a substrate. As these release techniques and methods, producing a cavity on a wafer substrate (referred to as “bulk micromachining”), removing a sacrificial layer formed on the middle part of the wafer substrate (referred to as “surface micromachining”), and the like, are known.
A widely used method for fabricating the MEMS structure involves wet etching the sacrificial layer to remove and release it. The method of fabricating may be described as follows.
That is, as shown in FIG. 1a, a Silicon on Insulator (SOI) wafer substrate 10 is fabricated by disposing a sacrificial layer, i.e., an oxide film 12 made of SiO2 between upper and lower silicon layers 11 and 13. Then, as shown in FIG. 1b, predetermined patterns are formed by printing a photoresist mask 14 on the upper surface of the wafer substrate 10 using photolithography.
Next, as shown in FIG. 1c, by dry etching, in the vertical direction from the upper surface to the lower surface, the wafer substrate 10 on which the patterns were not formed due to the photoresist mask 14, to form a plurality of etched holes 16. The depth of the etched holes 16 thus formed is controlled by stopping the etching process at the oxide film 12 which is a sacrificial layer.
Next, the photoresist mask 14 of an organic material remaining on the upper surface of the wafer substrate 10 having a plurality of the etched holes 16 formed thereon, as shown in FIG. 1d, is removed by an ashing process. A wet etchant such as HF, dilute HF (DHF), buffer oxide etchant (BOE), or the like is applied to the etched holes 16 on the wafer substrate 10 from which the photoresist mask 14 had been removed.
In this case, when the oxide film 12, which is a sacrificial layer formed between the upper and lower silicon layers 11 and 13 by wet etching using the etchant, is removed to form a floating space R corresponding to the height by which the sacrificial layer had been removed, as shown in FIG. 1e, there is fabricated a MEMS structure 1 having a stationary portion 1a in which the upper and lower silicon layers 11 and 13 are interconnected because the oxide film 12 has not been removed by the etchant, and a movable portion 1b floated on the floating space R from which the oxide film 12, which is a sacrificial layer, has been removed.
As can be seen from FIG. 2, the movable portion 1b movable relative to the stationary portion 1a horizontally extends from the stationary portion 1a and may take a beam or plate-like shape having upper and lower surfaces. An electrode portion 19 is formed on the upper surface of the stationary portion 1a. 
However, in order to remove the oxide film 12 in the process for preparing the MEMS structure 1 by using such a conventional method as described above, it is usually required to use the etchant. Further, there is employed a rinse solution in a rinse process which performs washing of the wafer substrate 10. Therefore, there may remain solutions such as etchant and rinse solution in the floating space R from which the sacrificial layer oxide film 12 has been removed.
Consequently, the solutions remaining in the floating space R exert surface tension on the movable portion 1b floated in the floating space R upon drying and removing them as show in FIG. 2, thereby a floating portion 20 of the movable portion 1b subsides downwardly and then the lower surface thereof adheres to the lower silicon layer 13 or other structures adjacent thereto. This is called “stiction” which interferes with driving of the MEMS structure 1, thus greatly deteriorating characteristics of the products.
Such a stiction phenomenon in the MEMS structure 1 lowers sensitivity of the sensor of interest. Further, where it is severe, there is difficulty in manufacturing the device, presenting a factor of decreasing yield of the micromachining process.
Recently, a variety of techniques for preventing the stiction phenomenon from occurring in the MEMS structure have been developed. One method is to minimize the contact area between the floating portion 20 of the movable portion 1b and the lower silicon layer 13. Another method is to solidify the solution that is a main factor of the surface tension between the floating portion 20 of the movable portion 1b and the wafer substrate, followed by sublimation.
However, the conventional methods to prevent the stiction phenomenon from occurring in the MEMS structure 1 have disadvantages such as decreased yield, additional steps in a manufacturing process and expensive equipment.
Further, under the condition in which opposite electrical polarities are applied to the movable portion 1b and the lower silicon layer 13, when the floating portion 20 of the vertically deforming movable portion 1b is in contact with the wafer substrate, electrical short circuit is induced thus causing product defects leading to disabling function of the sensor.
Additionally, the height of the floating space R thus formed is restricted by the thickness of the oxide film 12 made of an expensive material. Thus, if it is desired to elevate the height of the floating space R, so as to increase the range of motion of the movable portion 1b, there is a defect of increased costs in manufacturing the MEMS structure 1.